1. Field of the Invention
The present invention relates generally to a fluid-filled type vibration-damping device for connecting in a vibration-damping fashion a vibration damping member with a member to be vibration-damped. More particularly, the invention is concerned with a fluid-filled type vibration-damping device having a flexible film as a fluid pressure absorbing mechanism.
2. Description of the Related Art
Rubber vibration dampers having a first mounting member and a second mounting member connected by a main rubber elastic body are widely employed in a variety of fields as vibration-damped connectors or vibration-damped supports for installation between members making up a vibration transmission system. As one type of rubber vibration damper of this kind, there have been proposed fluid-filled type vibration-damping devices that utilize resonance or other flow action of a non-compressible fluid sealed within the device. As such devices, there are known fluid-filled type vibration-damping devices that typically comprise: a first mounting member and a second mounting member attached respectively to a vibration-damped member and connected to each other by a main rubber elastic body; a pressure-receiving chamber whose wall is partially constituted by the main rubber elastic body and that gives rise to pressure fluctuations upon input of vibration; an equilibrium chamber whose wall is constituted by a flexible film and which allows change in volume; a non-compressible fluid sealed within the pressure-receiving chamber and the equilibrium chamber; and an orifice passage permitting a fluid communication between the two chambers.
With such fluid-filled type vibration-damping devices, it is possible to achieve high vibration damping capability in the frequency range to which the orifice passage has been previously tuned. However, upon input of vibrations in a frequency range higher than the tuning frequency, the orifice passage becomes substantially blocked off, which can result in considerably increased dynamic spring constant, thereby making it difficult to exhibit adequate vibration damping performance.
In order to handle vibrations in a frequency range higher than the tuning frequency of the orifice passage, there has been proposed a fluid-filled type vibration-damping device having a flexible, movable film disposed within the pressure-receiving chamber or an intermediate chamber that communicates with the pressure-receiving chamber through an orifice passage. In this fluid-filled type vibration-damping device, fluctuations in fluid pressure caused by medium- to high-frequency vibration is absorbed through elastic deformation of the movable rubber film, so that the system can handle vibration in a frequency range higher than the tuning frequency of the orifice passage.
In a fluid-filled type vibration-damping device having a movable rubber film, there is a risk that elastic deformation of the movable rubber film may also absorb even pressure fluctuations produced within the pressure-receiving chamber during input of vibration of the frequency range to which the orifice passage is tuned. As a result, vibration-damping capability based on fluid flow through the orifice passage may be diminished.
To address this problem, there have been proposed pneumatic dynamic vibration damping devices like that taught in JP-A-11-264436, wherein a movable rubber film (movable rubber plate) is disposed so as to define at one face within the pressure-receiving chamber or equilibrium chamber with non-compressible fluid sealed therein, and to define at the other face an air chamber. This arrangement makes it possible to control elastic deformation of the movable rubber film by controlling the pressure within the air chamber from the outside.
More specifically described, in the case of input of vibration in the frequency band to which the orifice passage has been previously tuned, negative pressure acts on the air chamber and suctions the movable rubber film, thus impeding its elastic deformation and displacement. Whereas in the case of input of vibration of a higher frequency than the tuning frequency of the orifice passage, the air chamber is maintained at substantially atmospheric pressure so as to permit elastic deformation and displacement of the movable rubber film to absorb fluid pressure associated with input of vibration.
Extensive studies conducted by the inventors, however, has revealed that the use of such a movable rubber film creates a new problem. That is, where the movable rubber film is made sufficiently pliable with reference to the required characteristics of the movable rubber film in order to produce a low dynamic spring constant, the movable rubber film may suffer from insufficient elasticity. In this state, the movable rubber film does not recover its original shape prior to suction after the suction created by the negative pressure is released, and cannot exhibit the desired performance.
Additionally, where a fluid-filled type vibration-damping device like those described above is employed in an automobile or the like, it is typical to employ the exhaust system of the internal combustion engine as the means for controlling pressure in the air chamber. However, since there is backflow of gas and oil from the combustion chamber end into the air intake passage of the internal combustion engine, there is a risk that gas or oil will flow into the air chamber of the fluid-filled type vibration-damping device. Thus, oil resistance is required of the movable rubber film which makes up part of the wall of the air chamber, making it preferable to use as the material for the movable rubber film a material such as containing added hydrogen in order to meet the requirement of oil resistance. However, movable rubber films produced from such materials are relatively stiff, and where it is attempted to impart the desired characteristics, it becomes difficult to provide thickness adequate to ensure a sufficient level of durability.
On the other hand, where the movable rubber film using materials like those described above to impart oil resistance and thickness sufficient to ensure durability, the movable rubber film becomes highly rigid, which not only makes it difficult to meet the requirement of low spring characteristics. In addition, due to the lower pliability also poses a risk of rupture or other damage due to repeated elastic deformation by negative pressure suction. Accordingly, there is a great demand for a fluid-filled type vibration-damping device furnished with a movable rubber film that simultaneously achieves sufficient rigidity, oil resistance, and durability.